This invention is directed to an improved method for compression impregnating wood veneer with water borne chemical agents. More specifically, this invention is directed to the treating of wood veneers comprising passing multiple layers of the veneer between compression rollers while submerged in a solution of the treating compound. The veneers are fed into the compression rollers parallel to the grain or cell orientation and are compressed by the rollers perpendicular to the grain to about one-half of their original thickness. The veneers have an original moisture content above the fiber saturation point before compression.
It is well known in the art that plywood is a superior wood product when compared to other wood products. Plywood has near uniformity in strength properties in the direction of its two major axes and has increased resistance to end checking and splitting and a greatly reduced tendency to twist and warp. This superiority of plywood is largely due to the manner in which plywood is constructed. Wood is an exceedingly anisotropic material with its flexural strength perpendicular to the grain being about one-fourth of the flexural strength in the direction of the grain. Similarly, the shrinkage and swelling of wood are highly directional. Plywood, by changing the grain direction of successive layers by 90.degree., minimizes these variations. Therefore, plywood is a highly desirable wood product.
Because of the desirability of plywood it is becoming more and more common and desirable in the industry to treat plywood with fire retarding chemicals, preservatives, stabilizers and other wood treating agents. However, the present method of treating plywood at pressure treating plants requires several very costly handlings, a kiln drying operation which often results in undesirable sticker marks and other damage, and a process time of 30 days or longer. Because of these drawbacks treated plywood is not as economically attractive as would be desirable.
It is obvious that there would be a large economic advantage in being able to manufacture treated plywood from veneers which have been treated as they came from the veneer lathe and before drying. Such a procedure would produce treated plywood without sticker marks and other defects associated with pressure treatment and kiln drying. In addition, such a procedure would not require the costly handling steps and the long process times. The treated plywood could be produced at minimal cost right at the plywood mill where it could then be shipped through normal channels to customers. However, heretofore there was not available to the art an effective and economical method for treating green veneers as they came from the lathe.
It has been proposed in the past to treat veneers by passing a single layer of the veneer through pressure rollers while submerged in a solution of the treating agent. See, for example, Stamm, U.S. Pat. No. 2,350,135. However, the Stamm process was never commercialized because of several severe drawbacks. First of all, the Stamm process required the veneers to be preheated prior to passing them between the pressure rollers. The wood must be plasticized by first heating it to about 200.degree.-300.degree.F. This step is expensive and time consuming and adds appreciable costs to the process. In addition it is incompatable with most preservative and fire retarding agents.
Another drawback of the Stamm process is that the green veneer must be presqueezed to remove water from the green veneer before it is passed through the compression rollers which are submerged in the treating solution. According to Stamm, the wood must not have water concentrations greater than the fiber saturation point. If the water concentration is greater than the fiber saturation point, then Stamm teaches that both the amount and rate of solution takeup are reduced. Most green veneer have water concentrations which are appreciably above the fiber saturation point. Therefore, it is necessary in the Stamm process to pass the green veneer through a pair of hot drying rollers to squeeze out the free water (above fiber saturation point) prior to passing the veneer through the rollers located in the treating bath. It is also necessary to remove this excess water from the presqueezer by blowing hot air over the wood surface at the point where the veneer passes between the rollers of the presqueezer. This treatment step is also costly and time consuming.
The most serious drawback of the Stamm process is that it seriously degrades the veneer during the squeeze treatment. When the veneer passes between the squeeze rollers of the Stamm the knots in the wood are crushed and damaged. As the knots are crushed, they split out and rupture the veneer; quite often the wood around the knot is also seriously damaged. Since it is well known that there are knots present in all veneers, this presents a serious problem. The higher grades of veneer (N and A) are degraded to grades C and D. In addition, some of the veneer cannot be used because of the damage.
The knot occurence in veneer depends on the species from which the veneer is cut, the grade of logs peeled and the depth in the log from which the veneer came. In some species there will be several knots per square foot whereas in others there will be only several knots in a 4 feet .times. 8 feet sheet. However, there are very very few sheets without knots. For example, veneers cut from spruce logs contain a very large number of knots often averaging several per square foot of veneer. These knots are small but cruse very easily when the sheets of veneer are compressed between steel rollers. Southern pine veneers also contain a large amount of knots most of which are relatively large in size. The large knots are always damaged when compressed between hard rollers. Because of the knot problem and the other drawbacks mentioned above, the use of compression treatment with rollers has not heretofore found commercial use for the treatment of veneers.
For examples of other compression based processes which have been patented but never commercialized see: British Pat. No. 1,060,014; British Pat. No. 1,174,713 and Goulet, U.S. Pat. No. 3,624,233. In addition see Finnish Pat. No. 34,854 and Varga U.S. Pat. No. 3,429,652.
Therefore, it is the object of this invention to provide an effective and economical process for treating green veneer as it comes from the lathe.